1. Field of the Invention
The present invention relates to an improvement of a rear discharge mower of a lawn mower, and a mid-mount type lawn mower that uses the rear discharge mower.
2. Description of the Related Art
This type of mid-mount type lawn mower that uses a rear discharge mower has, e.g., the following conventionally known structure.
A lawn mower is known in which a power transmission pulley and a power transmission belt are used as means to transmit power to a plurality of adjacently positioned blade shafts, in which one of the blade shafts and the shaft of the power transmission pulley for providing input to the blade shaft are structured as separate shafts in order to drive the two shafts in mutually reverse directions, and in which a set of gears is provided to ensure reverse rotation between the two shafts (see Japanese Laid-Open Patent Application (JP-A) 2003-274728 (Patent Document 1), for example).
A lawn mower is also known in which a power transmission pulley and a power transmission belt are used as means to transmit power to a plurality of adjacently positioned blade shafts, and in which a supplementary pulley is used to convert the winding direction of the belt so that the internal peripheral surface side of the power transmission belt is wound around the power transmission pulley on one of the blade shafts, and the external peripheral surface side of the power transmission belt is wound around the power transmission pulley of another blade shaft, in order to drive the two shafts in mutually reverse directions (see Japanese Laid-Open Patent Application (JP-A) 2003-274729 (Patent Document 2), for example).
A lawn mower is further known in which motive power transmitted from an input shaft is branched at a pair of power transmission shafts to provide input to a gearbox disposed in the upper portion of left and right blade shafts and to transmit shaft power via a bevel gear mechanism or the like as a means to pivotally transmit power to a plurality of blade shafts in adjacent positions (see Japanese Laid-Open Patent Application (JP-A) 2003-158907 (Patent Document 3), for example).
In these kinds of rear discharge mowers, the blades are driven at a fairly high speed and high torque in order to discharge grass clippings in a rearward direction by generating airflow and producing a bouncing action that accompany the rotation of the blades.
If an attempt is made to convey the grass clippings toward the grass catcher in the rearward and upward directions while minimizing a reduction in the movement energy in the grass clippings discharge direction, it is desirable to gradually change the movement of the grass clippings upward from the conveyance start area in which a conveyance action is imparted by the rotation of the blades across to the location where discharge begins on the rearward side where the conveyance action is completed. In order to accomplish this, it would be advantageous to gradually change the movement direction by beginning an upward slope of the grass-clippings discharge path from a position as near the front end of the mower deck as possible.
In models having a structure in which motive power is transmitted to the blade shafts using a belt as described in Japanese Laid-Open Patent Applications (JP-A) 2003-274728 and 2003-274729 noted above, the belt is used in a state of high belt tension. Therefore, the strength of the blade shaft support structure and the mower deck must be kept very high because the provision of a large distance between the mower deck and the power transmission pulley results in a situation in which a strong tension operates on the power transmission pulley in a position set at a distance from the mower deck, and there is a possibility that a bending moment will forcefully operate on the blade shaft. However, this is not practical in actual application in a mower deck composed of a thin metal plate because sufficient support strength cannot be obtained and the thickness of the mower deck must be unavoidably increased to assure strength, and other impractical structural modifications become necessary when attempting to set the power transmission pulley at a distance from the mower deck.
For this reason, in the arts described in Japanese Laid-Open Patent Applications (JP-A) 2003-274728 and 2003-274729 noted above, measures are taken in which the power transmission pulley is not placed at a distance from the mower deck, but rather a supplementary pulley is used to bring the power transmission belt between adjacent blade shafts to the front side of the mower deck to the extent possible, and the location at which the grass-clippings discharge path portion on the mower deck begins an upward movement is set further forward. However, since there is no change in the fact that the power transmission pulley remains in close proximity on the surface of the mower deck, the location at which upward motion starts must be provided further rearward to avoid [close] placement of the power transmission pulley and belt, and the location at which upward motion starts cannot be brought sufficiently forward.
Moreover, insofar as a belt is used as the transmission mechanism, some sort of blade reversal mechanism must be used between adjacent blade shafts, necessitating a dedicated set of gears or a redundant supplementary pulley for that purpose.
A structure is also conventionally known in which power transmission is branched by using a pair of power transmission shafts, a bevel gear mechanism, or the like in place of a structure using a belt and power transmission belt, as described in Japanese Laid-Open Patent Application (JP-A) 2003-158907.
In a structure that drives a bevel gear about the center of a horizontal shaft using a power transmission shaft in this manner, there are no drawbacks such as the generation of high tension between adjacent blade shafts, but the following problems arise because placement of a bevel gear with a rather large diameter is difficult on a mower deck that has little extra space.
The power transmission mechanism itself, which involves the meshing of bevel gears with each other, produces a relatively high level of noise in comparison with a power transmission mechanism composed of flat gears. In addition, there is a drawback when the bevel gear itself has a small diameter in that the noise in the gear meshing portion increases further due to the load that operates in a concentrated fashion on the meshing portion that has a small number of teeth. Also, a separate reverse rotation mechanism naturally becomes necessary.